There are several reasons why it is important to control the amount of thermal energy that a substrate receives during semiconductor device processing. For example, some of the structures that are formed at the various stages of processing are sensitive to thermal energy and tend to change over time as thermal energy is absorbed by the substrate. Further, many fabrication processes are sensitive to thermal energy and tend to produce varying results as the thermal energy absorbed by the substrate changes. Thus, if the amount of thermal energy absorbed by the substrate is not controlled, processes tend to produce unexpected or undesired results, and structures that are initially properly formed tend to degrade toward unexpected or undesired forms. Therefore, it is important to control the amount of thermal energy that the substrate receives during processing.
The amount of thermal energy absorbed by the substrate can be controlled in at least two different ways. First, the temperature at which the substrate is maintained can be controlled. For example, even if the substrate is in a hot environment, the substrate itself can be cooled so that the bulk of the substrate does not experience anything more than a given temperature. Second, the length of time at which the substrate is exposed to a given temperature can be controlled. By controlling these two parameters the amount of thermal energy absorbed by the substrate during processing can be controlled.
Unfortunately, it is difficult to control the temperature of the substrate during processing. Temperature conditions experienced by the substrate in some processes tend to vary according to one or more of a number of different parameters, such as position on the substrate, length of processing time, and processing energy. Traditional substrate temperature control systems tend to be ineffectual in controlling the temperature of the substrate when faced with the interactions between one or more of these and other thermal energy parameters.
What is needed, therefore, is a system for controlling the temperature of a substrate during processing that accounts for temperature variation parameters such as position, time, and energy.